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(11) | EP 1 312 816 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Composite bushing with a monolithic ceramic core |
| (57) A high temperature bushing 10 includes an inner core 12 made of a monolithic ceramic
material having a through-hole 14 formed therein of predetermined diameter; and an
outer housing 24 substantially encasing the inner core 12, the outer housing 24 made
of a ceramic composite matrix material. A method of making the bushing 10 the bushing
includes the steps of a) forming an inner core 12 of silicon nitride or silicon carbide
having a through-hole 14 of predetermined diameter; and b) applying a ceramic matrix
composite material over substantially all of the inner core 12. |
[0001] This invention relates to ceramic matrix composite components in general, and specifically, to a high temperature ceramic matrix composite bushing designed for attachment to lower temperature metallic gas turbine components.
[0002] Ceramic matrix composites offer high material temperature capability. In the gas turbine field, however, they often require attachment to lower temperature metallic gas turbine components. Problems associated with the attachment of known silicon carbide ceramic matrix composites (CMC's) to metallic components include wear, oxidation (due to ionic transfer with metal), stress concentration (from clamping loads), transition to thick section fabrication, and fiber damage in creating holes in the CMC's.
[0003] In an exemplary embodiment of the invention, advantage is taken of the very high strength of monolithic ceramics to absorb the clamping loads of bolt and pin-type attachment means. The bushing thus has an inner core made of either silicon carbide or silicon nitride monolithic ceramic that is embedded within an outer housing made of CMC material that is molded about the inner core. It is understood that material selection for the inner core depends on specific attachment requirements, and the shape of the inner core of the bushing could be any number of shapes, two of which are disclosed herein. Preferably, the shape of the inner core would thus be optimized to insure that the inner core is well encased within the CMC component, and that the load is optimally distributed into the CMC structure. In the exemplary embodiment, the required size of the hole in the inner core of the bushing will contribute to set the overall size of the bushing in order to preserve an appropriate surface area of monolithic ceramic within the CMC component.
[0004] Accordingly, in its broader aspects, the present invention relates to a bushing comprising an inner core comprised of a monolithic ceramic material having a through-hole formed therein of predetermined diameter; and an outer housing substantially encasing the inner core, the outer housing comprising a ceramic composite matrix material.
[0005] In another aspect, the invention relates to a bushing comprised of silicon nitride or silicon carbide, the inner core having a first outer diameter and a through-hole formed therein having a second diameter, the first diameter being 2.5 - 4 times the second diameter; an outer housing substantially enclosing the inner core, the outer housing comprised of a ceramic matrix composite material.
[0006] In still another aspect, the invention relates to a method of making a bushing for use in gas turbine constructions comprising a) forming an inner core of silicon nitride or silicon carbide having a through-hole of predetermined diameter; and b) applying a ceramic matrix composite material over substantially all of the inner core.
[0007] The invention will now be described in greater detail, by way of example, with reference to the drawings, in which:-
FIGURE 1 is a side elevation of a bushing having a monolithic ceramic core encased within a ceramic matrix composite material in accordance with an exemplary embodiment of the invention;
FIGURE 2 is a side view of the bushing shown in Figure 1; and
FIGURE 3 is a side view of a bushing in accordance with an alternative embodiment of the invention.
[0008] With reference to Figures 1 and 2, the bushing 10 includes a monolithic ceramic inner core 12 made of either silicon nitride or silicon carbide. These materials bond well with the surrounding ceramic matrix composite matrix (CMC), while providing extremely hard and wear resistant surfaces. Monolithic ceramics also can maintain close tolerances needed for specific attachment requirements. The inner core 12 includes a through-hole 14 that is sized to receive a metallic gas turbine component such as a bolt shank or pin. The inner core 12 also includes flat, annular portions or faces 16, 18 that are flush with the opposite sides 20, 22 of the partially surrounding outer housing 24.
[0009] The inner core 12 is reduced in thickness in a radially outward direction in symmetrical fashion from the flat annular faces 16, 18 to an internal, maximum diameter curved edge 26 via substantially flat tapered surfaces 28, 30. Edge 26 is substantially centered between the sides 20, 22 of the CMC housing 24.
[0010] In an alternative embodiment shown in Figure 3, where similar reference numerals for corresponding elements are used, but with the prefix "1" added, the inner core 112 reduces in thickness in a radially outward direction from the flat annular faces 116, 118 to an internal, maximum diameter convex curved edge 126 via concave curved surfaces 128, 130.
[0011] In both cases, the manufacture of the bushing 10 is similar. Thus, after producing the silicon nitride or silicon carbide monolithic inner core 12, the CMC material is molded over the inner core 12. The manner in which this is done may be similar to the way in which components are embedded in fiberglass, with layer upon layer of the CMC laid up on the monolithic inner core in sections or strips, until the outer dimensions of the composite bushing are achieved.
[0012] The arrangement described above is intended to take advantage of the very high strength of monolithic ceramics to absorb the clamping loads of bolt and pin type attachments. The inner core 12 allows the stress field around attachment points to be spread out over a larger area supported by the surrounding fibers within the larger area CMC outer housing 24.
[0013] Material selection will depend on specific applications, and the shape of the inner core 12 of the bushing may be varied to provide optimum load distribution into the surrounding CMC outer housing 24. In addition, the diameter of the hole 14 determines in part the size of the bushing in order to preserve an approximate surface area of the monolithic inner core within the CMC outer housing. Preferably, a ratio of overall monolithic inner core diameter to hole diameter of 2.5 - 4 is maintained, the exact ratio being determined by specific application requirements.
[0014] For the sake of good order, various aspects of the invention are set out in the following clauses:-
1. A bushing (10) comprising:
an inner core (12) comprised of a monolithic ceramic material having a through-hole (14) formed therein of predetermined diameter; and
an outer housing (24) substantially encasing said inner core, said outer housing comprising a ceramic composite matrix material.
2. The bushing of clause 1 wherein said monolithic ceramic material comprises silicon nitride.
3. The bushing of clause 1 wherein said monolithic ceramic material comprises silicon carbide.
4. The bushing of clause 1 wherein said inner core (12) has front and rear faces (16, 18) about said through-hole (14) that are flush with opposite sides (20, 22) of said outer housing.
5. The bushing of clause 4 wherein said inner core (12) further includes surfaces (28, 30) that taper inwardly from said front and rear face portions to a curved, radially outer edge (26) centered relative to said opposite sides (20, 22) of said outer housing (24).
6. The bushing of clause 5 wherein said surfaces (28, 30) are substantially flat.
7. The bushing of clause 5 wherein said surfaces (128, 130) are concave.
8. The bushing of clause 1 wherein said outer housing (24) is molded around said inner core (12).
9. The bushing of clause 5 wherein said maximum diameter of said inner core (12) is between 2.5 and 4 times said diameter of said through-hole (14).
10. A bushing comprised of an inner core (12) made of silicon nitride or silicon carbide,
said inner core (12) having a first outer diameter and a through-hole (14) formed
therein having a second diameter, said first diameter being 2.5 - 4 times said second
diameter; and
an outer housing (24) substantially enclosing said inner core, said outer housing
comprised of a ceramic matrix composite material.
11. The bushing of clause 10 wherein said inner core (12) has front and rear faces (16, 18) about said through-hole that are flush with opposite sides (20), (22) of said outer housing.
12. The bushing of clause 11 wherein said inner core further includes surfaces (28, 30) that taper inwardly from said front and rear faces to a radially outer edge (26) substantially centered relative to said opposite sides (20, 22) of said outer housing (24).
13. The bushing of clause 12 wherein said surfaces (28, 30) are substantially flat.
14. The bushing of clause 12 wherein said surfaces (128, 130) are concave.
15. The bushing of clause 10 wherein said outer housing (24) is molded around said inner core.
16. A method of making a bushing (10) for use in gas turbine constructions comprising:
a) forming an inner core (12) of silicon nitride or silicon carbide having a through-hole (14) of predetermined diameter; and
b) applying a ceramic matrix composite material over substantially all of said inner core.
17. The method of clause 16 wherein, during step a), said inner core (12) is formed to have an outer diameter 2.5 - 4 times larger than said hole diameter.
18. The method of clause 17 wherein said inner core (12) is formed to include flat front and rear faces (16, 18) connected to a radially outer edge (26) by tapered surfaces (28, 30).
19. The method of clause 18 wherein said tapered surfaces (28, 30) are substantially flat.
20. The method of clause 18 wherein said tapered surfaces (128, 130) are concave.
21. The method of clause 18 wherein, during step b), said ceramic composite matrix material is applied in such a way that all of said inner core (12) is covered except for said flat front and rear faces (16, 18).
an inner core (12) comprised of a monolithic ceramic material having a through-hole (14) formed therein of predetermined diameter; and
an outer housing (24) substantially encasing said inner core, said outer housing comprising a ceramic composite matrix material.
a) forming an inner core (12) of silicon nitride or silicon carbide having a through-hole (14) of predetermined diameter; and
b) applying a ceramic matrix composite material over substantially all of said inner core.